Never tried a "big block" muffler on a 120, I do know that the 112 up family will fit on the 120si.
I like to close up the 'louvers" and put a side exit on mine.
The old Sachs Dolmar muffs are some of the easiest to mod.
The reason I like to close up the louvers, is because stock they tend to blow down on the tank. Makes a mess and tends to burn the tank.